There are various prior Z Tip Tilt (“ZTT”) devices for adjusting the height and parallelism of a semiconductor wafer in a semiconductor processing machine. ZTT devices typically control positioning of Z-axis displacement, rotation about an X-axis, and rotation about a Y-axis while the semiconductor wafer is moving in the X-Y directions under a semiconductor processing machine, such as an optical inspection system. The ZTT device dynamically compensates for non-flatness of the wafer and should be stiff to provide high bandwidth positioning.
Typical ZTT devices are mounted on an X-Y positioning stage and should be sufficiently lightweight and compact to maintain the dynamic performance of the X-Y stage. The ZTT positioning device should also be accurate within a few nanometers, be geometrically stable, and have a sensitive and repeatable driving system. Moreover, ZTT devices should prevent contact between the wafer and the processing system, should not generate particles that could contaminate the wafer, and should be sufficiently reliable to maintain wafer processing throughput.
A conventional approach for providing ZTT positioning integrates two or more separate technologies or products, such as mechanically splitting the Z-axis (vertical) positioning and the tip and tilt positioning, an approach which typically results in very large, high profile, high-mass mechanisms. When splitting the Z-axis and tip/tilt positioning, the most common approach maintains a fixed wafer Z-axis position and, instead, moves the wafer inspection/processing elements. This approach complicates the design of the inspection/processing elements (typically a multi-element optical assembly) and increases the risk of particulate contamination because the vertical translation stage is typically located directly above the wafer. Also, because the moving mass of the Z-axis translation stage (and the elements it carries) is greater than that of a wafer chuck, the resulting dynamic performance is inadequate for many high-throughput applications.
Another conventional approach also mounts the tip and tilt positioners above the wafer. A problem with this approach is maintaining co-location of the inspection/processing system focal point and the tip and tilt positioner axes to prevent X-Y translation of the inspection/processing point as the tip and tilt angles are changed. Of course, mass, complexity, and contamination risk remain problems with this over-wafer configuration.
Several conventional approaches exist for providing tip and tilt positioning beneath the wafer chuck, such as on the X-Y stage carriage. For example, stacking two goniometric cradle stages with coincident rotational axes provides tip and tilt rotation about a common point located at the wafer surface. This approach provides relatively large tip and tilt positioning angles but is problematic because it employs mechanical bearings and drive screws, has a high profile, and cannot directly measure the tip and tilt angles. Alternatively, this cradle approach may be further coupled to a Z-axis stage that is also located on the X-Y stage carriage. The most common conventional Z-axis stages for mounting to an X-Y stage employ either a horizontal wedge driven by a mechanical actuator or linear motor, a single drive screw with a vertical guide way, or three or four small vertical drive screws that turn synchronously to provide Z-axis movement. All these approaches are overly tall and massive to achieve suitable dynamic performance in high throughput applications.
Another conventional tip and tilt positioner approach employs flexure mechanisms driven by mechanical or piezo-electric actuators connected to a support plate that rests on a pivot point defining the center of tip and tilt rotation. In this approach, two identical flexures spaced apart by 90 degrees and at a same radius from the pivot point, provide rotation about one axis and translation along another axis. The combination of rotation and translation creates the tip and tilt positioning. However, the flexures must be compliant through the rotational axis while providing stiffness for the mechanical structure. This tradeoff limits either rotational range or stiffness.
Another conventional flexure approach employs a single stage that provides tip, tilt, and a small amount (less than 1 mm) of Z movement, by simultaneous actuation of two opposing flexures. This approach employs four flexures, a support plate, but no centered pivot point. The four flexures are spaced apart 90 degrees around the circumference of the support plate. Tip and tilt movement is provided by actuating two opposing flexures in opposite directions. Z-axis movement is provided by actuating all four flexures in the same direction. This approach also suffers from limited range or a lack of mechanical stiffness.
In addition to ZTT positioning, many wafer processing applications also require rotational angle (theta) positioning about the Z-axis. Theta positioning typically includes static “fine theta” adjustments for aligning a wafer when it is loaded on a chuck and “dynamic theta” adjustments for maintaining alignment during movements of the X-Y axis positioner. The fine and dynamic theta positioners are typically mounted on the X-Y positioning stage. The fine theta positioner should be close to the wafer to avoid X-Y errors, whereas the dynamic theta positioner should be mounted at a lower position to compensate for parasitic rotations of the wafer.
As with the ZTT positioners, the fine and dynamic theta positioners should be lightweight, compact, and stiff to provide suitable dynamic performances; accurate to within a few nanometers; stable, sensitive, and repeatable; should not generate wafer contaminating particles; and be sufficiently reliable to maintain machine throughput.
A common conventional theta positioner employs a mechanical rotary stage mounted to the X-Y positioning carriage. Such a rotary stage includes a rotating carriage supported by a worm-gear driven radial bearing set. Alternatively, a direct-drive torque motor may drive the stage. However, the mass, height, and inherent mechanical properties of the bearing stage compromise the X-Y stage performance. Moreover, achieving a desired zero-dither performance for the theta stage requires adding a brake or locking mechanism to the stage, which further increases the mass and complexity of the positioner.
A solution for providing suitable theta positioning performance employs a simple two-plate air bearing structure in which a flat reference plate is mounted to the X-Y stage carriage. An upper plate having pressure and vacuum orifices is installed above the reference plate forming an air bearing gap between the two plates. The upper plate is tangentially driven by a linear actuator on one end and is supported by a rigid flexure mechanism on the opposite end to form a pivot point for the theta adjustment. After adjustment, the air bearing pressure supply is blocked, allowing the remaining vacuum to adhere, and thereby lock, the upper and lower plates together. However, because the stage is locked, it cannot provide the dynamic theta adjustments required by some applications. Moreover, the travel range of this approach is limited by the rigid flexure mechanism and by a lateral shift that occurs between the actuator contact point. Another disadvantage of this approach is that the center of rotation is offset from the X-Y carriage center, making it necessary to compensate in X-Y for the theta offset angle.
A solution for providing both very fine theta adjustment within about one degree and high-bandwidth response employs differential positioning of two parallel stages connected by a single perpendicular stage. This approach, referred to as an H-bridge configuration, employs flexures at each end of the single perpendicular stage to allow a small amount of individual mechanical movement between two connected parallel stages. This movement creates an offset angle of the single stage with respect to the parallel axes and, in turn, the desired theta offset functionality. While this solution adds little hardware to the X-Y system to provide theta functionality, it still has a limited travel range and provides no way to lock the theta position. High-bandwidth theta adjustments are possible with the H-bridge configuration, but because flexures are needed to accommodate the differential movement of the parallel stages, the dynamic response of the X-Y stage is reduced by the flexure compliance.